Oxidation & Reduction [Free JAMB Tutorial on Chemistry]

This Chemistry Tutorial will focus on OXIDATION AND REDUCTION. This topic has been explained to ensure you understand it very well. At the end of the tutorial, you’ll be able to download it for FREE. Please share this page with your friends who may need it.

Meaning of Oxidation:

Oxidation is defined as a process in which a substance loses electrons. If an atom or ion goes from a lower oxidation state to a higher oxidation state or loses electrons in a reaction, it is undergoing oxidation.

If you didn’t understand that, imagine you have a tasty apple slice. When you leave it out in the air for a while, it starts to turn brown. That browning process is a bit like oxidation.

Oxidation is when something combines with oxygen or, in simple terms when things react with the air. So, when your apple slice reacts with the oxygen in the air and turns brown, that’s oxidation! Another example is when metal gets rusty – it’s oxidizing.

Now, think of oxidation as adding oxygen or losing electrons. It’s like your apple slice sharing with the air.

Meaning of Reduction:

While reduction is a process in which a substance gains electrons. If an atom or ion goes from a higher oxidation state to a lower oxidation state or gains electrons in a reaction, it is undergoing reduction.

Didn’t get that explanation? imagine you have a friend who gives you some of their toys. That’s like reduction – gaining something. In chemical terms, a reduction is often linked with gaining electrons.

Oxidation and reduction are always coupled in a reaction; when one substance is oxidized, another is reduced. Together, they make up a redox (reduction-oxidation) reaction.

Just remember “LEO says GER” (Lose Electrons in Oxidation, Gain Electrons in Reduction) to help recall the definitions.

Oxidation in terms of the addition of oxygen or removal of hydrogen.

Oxidation can be expressed in terms of the addition of oxygen or the removal of hydrogen in a chemical reaction.

1. Addition of Oxygen:
   • When a substance gains oxygen during a reaction, it undergoes oxidation.
   • Example: C+O₂​→CO₂​ Here, carbon (C) is oxidized because it gains oxygen (O₂​) to form carbon dioxide (CO₂​).
2. Removal of Hydrogen:
   • When a substance loses hydrogen during a reaction, it is also undergoing oxidation.
   • Example: CH₄​+2O₂​→CO_2​+2H₂​O In this reaction, methane (CH₄​) is oxidized because it loses hydrogen to form carbon dioxide (CO₂​) and water (H₂​O).

In both examples, you can see that the substances undergoing oxidation are gaining oxygen or losing hydrogen. These expressions are useful ways to recognize oxidation in chemical reactions, especially in the context of redox reactions where electrons are transferred.

Reduction as removal of oxygen or addition of hydrogen.

Reduction can be expressed in terms of the removal of oxygen or the addition of hydrogen in a chemical reaction.

1. Removal of Oxygen:
   • When a substance loses oxygen during a reaction, it undergoes reduction.
   • Example: 2Fe₂​O₃​+3C → 4Fe+3CO₂​ Here, iron(III) oxide (Fe₂​O₃​) is reduced because it loses oxygen to form iron (Fe) and carbon dioxide (CO₂​).
2. Addition of Hydrogen:
   • When a substance gains hydrogen during a reaction, it is also undergoing reduction.
   • Example: N₂​+3H₂​→2NH_3​ In this reaction, nitrogen (N₂​) is reduced because it gains hydrogen to form ammonia (NH₃​).

In both examples, you can see that the substances undergoing reduction are either losing oxygen or gaining hydrogen.

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Use of oxidation numbers.

Oxidation numbers (or oxidation states) are assigned to atoms in a chemical compound or ion to help keep track of electron distribution in chemical reactions.

Here are several key uses of oxidation numbers:

1. Identifying Oxidation and Reduction:
   • Oxidation numbers help identify which atoms are oxidized (undergo an increase in oxidation state) and which are reduced (undergo a decrease in oxidation state) during a chemical reaction. This is essential in recognizing and understanding redox reactions.
2. Balancing Redox Equations:
   • Oxidation numbers are crucial for balancing redox reactions. By assigning oxidation numbers to the reactants and products, you can determine the number of electrons transferred and balance the reaction accordingly.
3. Predicting Reaction Products:
   • Knowledge of oxidation numbers helps predict the possible products of a reaction. In many cases, the reactants undergo changes in oxidation states, and the resulting products can be inferred by examining these changes.
4. Deducing Oxidation States:
   • Oxidation numbers provide a systematic way to deduce the oxidation states of elements in compounds. Understanding the rules for assigning oxidation numbers allows chemists to determine the likely oxidation state of an element in a given compound.
5. Analyzing Electron Transfer:
   • Oxidation numbers are directly related to electron transfer in redox reactions. They indicate the number of electrons gained or lost by each atom. This information is valuable for understanding the flow of electrons and the mechanisms underlying chemical reactions.
6. Understanding Redox Processes:
   • In electrochemical processes and batteries, oxidation numbers help in comprehending the movement of electrons and the conversion of chemical energy into electrical energy. The knowledge of oxidation numbers is essential for designing and optimizing these systems.
7. Characterizing Compounds:
   • Oxidation numbers provide a useful shorthand for describing the distribution of electrons in a compound. They are often included in chemical formulas to convey information about the nature of chemical bonds and the overall charge of the compound.
8. Defining Formal Charge:
   • Oxidation numbers are used to calculate the formal charge of an atom in a molecule. The formal charge helps in understanding the distribution of electrons in a more detailed way than oxidation numbers alone.

IUPAC Nomenclature of Inorganic Compounds Using Oxidation Number

The IUPAC nomenclature for inorganic compounds using oxidation numbers involves assigning a systematic name to a compound based on the oxidation state (or oxidation number) of the elements present.

Here are some general guidelines:

Ionic Compounds:

1. Cations:
   • The name of the cation is the same as the element.
   • The oxidation state is indicated in Roman numerals in parentheses.
     • Example: Fe³⁺ is called iron(III).
2. Anions:
   • The name of the anion is derived from the stem of the element name with the suffix -ide.
   • Example: O²⁻ is called oxide.
3. Combining Cations and Anions:
   • The cation is named first, followed by the anion.
   • The overall compound should be electrically neutral.
   • Example 1: Fe³⁺ + O²⁻ is named iron(III) oxide.
   • Example 2: CuCl_2​ is named copper(II) chloride.

Covalent Compounds:

1. Identify the Two Elements:
   • Identify the two elements in the compound and determine their oxidation numbers.
2. Name the First Element:
   • Name the first element in the compound using its elemental name.
3. Name the Second Element:
   • The second element is named by taking the stem of the element name and adding the suffix -ide.
4. Indicate Oxidation Numbers:
   • Indicate the oxidation number of each element in parentheses using Roman numerals for metals with variable oxidation states.
   • Example 1: FeCl₃​ is named iron(III) chloride.
     • Iron is in the +3 oxidation state, and chlorine is in the -1 oxidation state.
   • Example 2: SO₂
     • First Element: Sulfur.
     • Second Element: Oxygen.
     • The compound is named sulfur dioxide.

Complex Ions:

1. Name the Cation First:
   • The cation is named first, followed by the anion.
   • For complex ions, the ligands are named first.
   • Example: Cu(H_2​O)₆​]²⁺ is named hexaaquacopper(II) ion.
2. Indicate Oxidation Numbers:
   • Indicate the oxidation number of the central metal ion in parentheses using Roman numerals.
   • Example: [Fe(CN)6​]³⁻ is named hexacyanidoferrate(III) ion.

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Tests for Oxidizing Agents

1. Reaction with Reducing Agents:
   • Oxidizing agents readily accept electrons. If you mix a potential oxidizing agent with a known reducing agent, you can observe a redox reaction.
   • Example: Reaction with hydrogen peroxide (H₂​O_2​) is often used. If a substance is an oxidizing agent, it will cause the hydrogen peroxide to decompose, releasing oxygen gas.
2. Reaction with Metals:
   • Some oxidizing agents can react with metals, leading to the oxidation of the metal. For example, an oxidizing agent might cause iron to rust or copper to undergo oxidation.
3. Color Changes:
   • Oxidizing agents can often cause color changes in certain indicators or solutions.
   • Example: Potassium permanganate (KMnO₄​) is a common oxidizing agent that changes color from purple to colorless when it undergoes reduction.
4. Bleaching Action:
   • Oxidizing agents often exhibit a bleaching action. They can oxidize colored substances, removing their color.
   • Example: Chlorine gas (Cl₂​) is a strong oxidizing agent and is used for bleaching.

Tests for Reducing Agents:

1. Reaction with Oxidizing Agents:
   • Reducing agents readily donate electrons. If you mix a potential reducing agent with a known oxidizing agent, you can observe a redox reaction.
   • Example: Reaction with a solution of iodine (I₂​) is commonly used. A reducing agent will cause the iodine to change color from brown to colorless.
2. Reaction with Oxygen:
   • Reducing agents can react with oxygen, causing a reduction reaction.
   • Example: Combustion of a substance. If a substance burns in the presence of oxygen, it is acting as a reducing agent.
3. Formation of Precipitates:
   • Some reducing agents can cause the reduction of metal ions to form precipitates.
   • Example: Reaction with silver nitrate (AgNO₃​). A reducing agent may cause the formation of a silver precipitate.
4. Hydrogen Evolution:
   • The evolution of hydrogen gas can indicate the reduction of a substance.
   • Example: Reaction with acids. Some reducing agents react with acids, producing hydrogen gas.

Calculations Involving Oxidation Number

Example 1: In the compound K₂​Cr₂​O_7​, what is the oxidation state of chromium?

In K₂​Cr₂​O_7​​, which is potassium dichromate, let’s determine the oxidation state of chromium (Cr).

First, we know that potassium (K) typically has an oxidation state of +1, and oxygen (O) usually has an oxidation state of -2 in compounds.

The sum of the oxidation states in a molecule or ion is equal to its overall charge.

For K₂​Cr₂​O_7​​, the overall charge is 0 because it’s a neutral molecule.

Let’s denote the oxidation state of chromium as x.

The equation to represent the sum of the oxidation states is:

2(+1)+2x+7(−2)=0

Solving for x:

2+2x−14=0

2x − 12=0

2x=12

x=+6

So, the oxidation state of chromium in K₂​Cr₂​O_7​​ is +6.

Example 2: What is the oxidation number of sulfur in the ion SO₄²⁻?

In the sulfate ion SO₄²⁻, let’s determine the oxidation state of sulfur (S).

Let’s denote the oxidation state of sulfur as x.

The sum of the oxidation states should equal the overall charge of the ion, which is -2.

The equation to represent the sum of the oxidation states is:

x + 4(−2) = −2

Solving for x:

x − 8 = −2

x = +6

So, the oxidation state of sulfur in the sulfate ion SO₄²⁻ is +6.

That’s all this tutorial will cover.

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Practice Questions

1. What is the oxidation state of sulfur in H₂SO₄?

A) +2
B) +4
C) +6
D) -2

2. Which of the following is an oxidizing agent?

A) Na
B) Cl₂
C) H₂O₂
D) Fe

3. What is the IUPAC name for FeO?

A) Iron(I) oxide
B) Iron(II) oxide
C) Iron(III) oxide
D) Ferrous oxide

4. Which of the following substances acts as a reducing agent in a reaction?

A) Cl₂
B) O₂
C) Na
D) H₂O₂

5. What happens when an oxidizing agent reacts with a reducing agent?

A) Release of heat
B) Precipitate formation
C) Redox reaction
D) Acid-base reaction

6. In which compound is nitrogen in the highest oxidation state?

A) NH₃
B) NO₂
C) NO₃⁻
D) N₂

7. What is the oxidation state of chlorine in Cl₂O₇?

A) -1
B) +1
C) +7
D) -7

8. Which compound is a complex ion?

A) KCl
B) [Fe(CN)₆]³⁻
C) H₂SO₄
D) CO₂

9. What happens when a reducing agent reacts with iodine solution?

A) Blue coloration
B) Brown coloration
C) Colorless solution
D) Red coloration

10. Which of the following compounds is an oxidizing agent?

A) HCl
B) Na₂SO₃
C) CH₄
D) HNO₃

11. What is the IUPAC name for CuO?

A) Copper(I) oxide
B) Copper(II) oxide
C) Cupric oxide
D) Cuprous oxide

12. What is the oxidation state of manganese in KMnO₄?

A) +2
B) +4
C) +6
D) +7

13. Which substance is commonly used as a reducing agent in organic chemistry?

A) H₂O₂
B) LiAlH₄
C) K₂Cr₂O₇
D) Cl₂

14. What is the oxidation number of nitrogen in NH₃?

A) +1
B) -1
C) 0
D) -3

15. What gas evolves when a reducing agent reacts with an acid?

A) Oxygen
B) Nitrogen
C) Hydrogen
D) Carbon dioxide

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